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Wednesday, 22 June 2011

Well, finally a day when I can spend some time at play here in the shack; the next part of the project has to be the beginnings of the receiver front end. So, in true form, and trying to stay faithful to the articles in RadCom Homebrew, I've moved on to the 1st RF mixer and the diplexer that follows it. The basic concept of a diplexer is really a band pass filter. Post the RF mixer there will be a mix of the RF input (through a bandpass filter from an antenna) and the LO. My LO isn't yet built but I have a DDS VFO here which will do just nicely. My IF is already set at 9MHZ, but the RadCom articles are assuming an IF of 10.7 MHz so I have some maths to do.
There are loads of on-line references to diplexer design, but I found this to be a particularly good starting point:

Monday, 20 June 2011

I've been testing the Low Level Amplifiers from my last post. There is something very strange going on! The first amp, from the RadCom article, seems to have some pretty good gain:

The input signal is at 14.14 MHz and an amplitude of about 300mV, here the output is clearly looking OK and has an amplitude of about 17V pk-to-pk. That's a gain of 20 log (17/0.3) which is about 35dB - exactly on the nail with the design gain. However, as soon as I hook another scope probe on the input to compare the two signals, the circuit seems to oscillate and some of the gear in the shack goes a bit nuts. The signal generator display starts to ramp up and down frequency as does the voltage display on my bench PSU, I suspect there's just too much RF flying about. This is the ouput signal with no scope probe on the input:

Sunday, 19 June 2011

Well there's not been a great deal of progress within the shack recently, chief technical assistant is of little or no use most of the time, and he does insist on sitting on the most sensitive bits of RF engineering. That plus the fact that my job has been getting rather badly in the way of play time means I've made little or no progress recently.

I've been trying to make the Low Level Amplifier as part of my ongoing attempt to replicate the SSB transceiver being built in the Homebrew articles in RadCom. This is the schematic of the LLA, published in Feb 2011:

My build of the circuit is here:

Now, if anyone is actually paying any attention to this blog of mine, or anyone happens to come across this post, I'd be grateful for any comments on my construction pictured above. All I am finding with this LLA is that it oscillates at the slightest whim. I've started to blame my RF signal generator (yes that's homebrew also), and various other aspects including the capacitance in my x10 scope probes. But I've just decided to find an alternative. Now the author of the fantastic homebrew column also has a website, Ed - EI9GQ has an alternative circuit on his site here: